1、 VOLUME 14, NUMBER 5 HVAC Jung et al.2000). Isobutane (R-600a) has dominated the European refrigerator/freezer sector for the pastdecade and is being used even in Japan and Korea, while propane (R-290) and propylene(R-1270) are used for heat-pumping applications in Europe (IEAHPC 2002). It is well k
2、nownthat hydrocarbons offer low cost, availability, compatibility with the conventional mineral oil,and environmental friendliness.As shown in Table 1, ASHRAE recently released a list of many environmentally friendlyrefrigerants (ASHRAE 2007). Most of them contain hydrocarbons and dimethyl ether (DM
3、E,RE170) as the primary components. Hydrocarbons and DME have very low GWP, typically lessthan 3, as compared to 1300 and 1700 of HFC134a and HCFC22, respectively. Due to climatechange all over the world, refrigerants with low GWP are in great demand these days and arecertainly good candidates for f
4、uture use. Furthermore, unlike pure hydrocarbons, these newrefrigerants are claimed to be “drop-in” replacements, with little change in the system in therefrigeration conversion process.Typically, refrigeration and air-conditioning applications are classified according to predomi-nant refrigerants,
5、such as HFC134a(CFC12), HCFC22, and R-404A(CFC502). Even thoughCFCs have been phased out completely in the developed countries, they are still being used inDongsoo Jung is a professor in the Department of Mechanical Engineering, Inha University, Incheon, Korea.2008, American Society of Heating, Refr
6、igerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC however, it is not being used inEuropean countries and cannot be used in new equipment produced after 2010 in the UnitedStates. This means that virtually all countries have to use environmentally safe refrigerants int
7、wo years. Some HFC alternatives have been used to replace HCFC22 over the past decade.European countries, however, are reluctant to use man-made chemicals for their refrigerationequipment and prefer using natural (or low global warming) fluids. R-431A and R-433A arepossible alternatives for HCFC22.
8、One residential air-conditioner manufacturer tested R-431Ain one of its commercial products and observed that R-431A had a 10% increase in COP with asimilar capacity as compared to HCFC22 (Song 2008).Finally, R-404A(CFC502) is used in low-temperature and transport refrigeration equipment.Once again,
9、 the world needs, if possible, non-HFC solutions in this area as well. The situation inthis application is quite similar to that of HCFC22. R-432A, R-433A, and R-433C may be goodalternatives in this application. Even though hydrocarbon- and DME-based new “drop-in” refrigerants offer high energy effi
10、-ciency with significantly less global warming, they have one common problemflammability.For hermetic systems, such as domestic refrigerators and water purifiers, flammability is not aserious problem, and refrigerators charged with isobutane (R-600a) have been predominant inthe world market except i
11、n the United States over the past several decades. As for residential andmobile air conditioners, however, flammable refrigerants have generated severe resistance fromthe manufacturers. The whole world is now faced with energy and environmental problems andis in need of a long-term global solution.
12、At this time, pure and mixed hydrocarbons and DMErefrigerants seem to be the best solution to the refrigeration industry as long as the flammabilityissue is tackled properly. One of the ways to overcome the flammability is the use of a secondary loop system by whichthe refrigerant is isolated to the
13、 outside of the working environment. Then the refrigerant will beseen as natural gas supplied to the kitchen for cooking. In fact, this system has already been wellexplained for mobile air conditioners with a prototype manufactured and demonstrated in anactual car by one of the worlds largest automo
14、bile manufacturers (Hill 2007). We live in an innovative technology age, and it is believed that the flammability issue can beovercome with the present engineering art and design technology. The one issue that remains,however, is the need to change our mindset. Unless there is a change in the way we
15、 think, wemay try in vain to develop more and more man-made chemicals but fail to solve this globalproblem. Are we ready for a change to utilize our natural resources to overcome the globalenergy and environmental problems with present technology in the refrigeration industry? Itmay be true that tom
16、orrows solution is here already in our hands today.634 HVAC accepted May 12, 2008A combination of factorsnotably environmental concerns about global warming and ozonedepletion due to refrigerants and the increasing demand for electronics and optoelectroniccoolingled to renewed activity in alternativ
17、e cooling technologies. Currently, thermoelec-tric cooling is considered a popular cooling technology. This paper provides a critical reviewof thermoelectric technology and assesses its potential applications in air conditioning andrefrigeration. The first part of this paper is devoted to the basic
18、concept of thermoelectrics,with an overview of current thermoelectric materials and devices. The second part is a gen-eral overview of the applications of thermoelectric technology, with an emphasis on thoserelated to air conditioning and refrigeration.INTRODUCTIONBecause of environmental concerns s
19、uch as global warming, ozone depletion, and a lack ofenergy efficiency, it is necessary to investigate alternative cooling technologies to the refrigera-tion that uses refrigerants 1. Thermoelectric cooling and heat pumping are alternatives thathave recently attracted attention. Thermoelectric devic
20、es are solid-state devices in which elec-trons or holesequivalent to refrigerants in traditional vapor-compression systemscarry elec-tricity and thermal energy under an electric field 28. Therefore, they have many inherent,attractive features such as a long life and no moving parts, and they dont em
21、it toxic gases, arelightweight, are low-maintenance, and are very reliable. In the past decade, there has been rapiddevelopment when it comes to the fundamental theory, materials, and devices related to thermo-electrics. This paper provides a critical review of thermoelectric technology and assesses
22、 itspotential applications in air conditioning and refrigeration. It should be noted that the informa-tion in this paper is influenced by the research focus of the present authors and reflects theirassessment of the field.The remainder of this paper is structured as follows: the next section provide
23、s the fundamen-tals of thermoelectric technology, and then the potential application of thermoelectric technol-ogy to air conditioning and refrigeration will be discussed. FUNDAMENTALS OF THERMOELECTRIC TECHNOLOGYIn this section, thermoelectric effects will be discussed first, then the overview of t
24、hermoelec-tric materials, modules, and systems will be presented.Bao Yang is an assistant professor and Herwin Ahuja is a graduate student at the Center for Environmental Energy En-gineering, Department of Mechanical Engineering, University of Maryland, College Park, MD. Thanh N. Tran is a sci-entis
25、t at the Naval Surface Warfare Center, Carderock Division, West Bethesda, MD.2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVACchemical potential due to the concentration is balanced by the built-in electrostatic potential,namely
26、 the Seebeck voltage. The Seebeck coefficient of the conductor is defined as,with a positive value when the electrical carriers are holes. Thermoelectric power generators arebased on this phenomenon. Note that the Seebeck coefficient is sometimes called the thermalEMF coefficient or thermoelectric p
27、ower.Peltier Effect. The thermoelectric cooling phenomenon is physically based on the Peltiereffect, which was discovered by Jean Peltier in 183413 years after the Seebeck effect wasunveiled 10. The Peltier coefficient is a measure of the amount of heat carried by electrons orholes. This amount of h
28、eat is proportional to the electrical current flowing in the circuit. Theproportionality constant is defined as the Peltier coefficient,where Q is the heat current and I is the electrical current.When two different materials are joined together to form a loop, as shown in Figure 2, therewill be an a
29、brupt change in heat flow at the junctions because the two materials have differentPeltier coefficients. The excess energy released to the lattice causes heating; the deficiency inenergy supplied by the lattice causes cooling. An interesting consequence of this effect is thatthe direction of heat tr
30、ansfer is controlled by the polarity of the electric current. Reversing theFigure 1. Seebeck effect.T T2T1()= T TcoldThot()=V V2V1()=VT-= QI=VOLUME 14, NUMBER 5, SEPTEMBER 2008 637electric polarity will change the direction of transfer and, thus, the sign of the heatabsorbed/evolved. The Peltier eff
31、ect is the principle at work behind thermoelectric modules (also called Peltiercoolers) or refrigerators that are used for transferring heat from one side of the device to theother.Thomson Effect. The Thomson effect describes the heating or cooling of a current-carryingmaterial subject to a temperat
32、ure gradient and was discovered by William Thomson (Lord Kel-vin) in 1851 11. Any current-carrying conductor with a temperature difference between twopoints will either absorb or emit heat, depending on the material. The Thomson coefficient isdefined as,where dQ/dx is the rate of the heating per uni
33、t length, I is the electrical current, and dT/dx is thetemperature gradient. Kelvin Relations. It is of great importance in thermoelectric theory that there exist thermody-namic relationships between these thermoelectric coefficients, called the Kelvin Relations orThomson Relations 12:Thermoelectric
34、 Element 4, 6, 1316An element of a thermoelectric module consists of p and n branches, as shown in Figure 3.When a current I flows through this thermoelectric element, the total heat flow, Q, within eachbranch (p or n) is expressed as:Figure 2. Peltier effect.dQdx- IdTdx-=T= Td dT=QppTI pApdTdx-=Qnn
35、TI nAndTdx-=638 HVAC accepted June 9, 2008The audience for this paper includes researchers, educators, and engineers in the fields of airconditioning, atmospheric physics, meteorology, psychrometrics, standards, and thermody-namics. This paper provides a brief history of the molar mass of dry air, M
36、da, followed by thecomposition of Earths atmosphere for the year 2008 and the calculation of Mda. A singleequation is given to calculate Mdabased on the actual abundance of CO2in the atmosphere,which is currently increasing at an annual rate of 1.9 molmol1. This causes an increase inthe value of Mda
37、 at a rate of 0.0001 kgkmol1(lblbmol1) for every 8.33 molmol1increasein the abundance of CO2. It is practical for many calculations to use the average projectedMdavalue over a period of a half-century, during which time the value of Mdawill increase byapproximately 0.0010 kgkmol1 (lblbmol1).For most
38、 psychrometric calculations, theauthors recommend an Mdavalue of 28.966 kgkmol1 (lblbmol1), which is projected for theyear 2036. This value will be correct when rounded to three decimal places through 2058 ifCO2increases at its current rate. INTRODUCTIONMdaVALUES FROM 1945 TO 2005Researchers, practi
39、tioners, and educators in the fields of agricultural and food science engi-neering, air conditioning, atmospheric physics, drying and dehumidification, gas turbines, com-pressors and expanders, meteorology, psychrometrics, and standards make numerouspsychrometric (moist air) calculations that are ba
40、sed in part on the molar mass of dry air. Dry airis a mixture of nitrogen, oxygen, argon, CO2, and eight or more minor constituents called tracegases. The molar mass of dry air is calculated as the sum of the products of the mole ratio ofeach gas times its molar mass.In the last half of the twentiet
41、h century, the following changes took place that resulted in anincrease in the molar mass of dry air: The scientific community changed from the Oxygen-16 to the Carbon-12 reference for themolar mass of elements and compounds in 1960.The molar masses of the basic chemical elements were updated by the
42、 International Union ofPure and Applied Chemistry (IUPAC) (Wieser 2005).CO2in the atmosphere has increased from 314 molmol1(1955) to 379 molmol1(Keel-ing and Whorf 2005a, 2005b). The 65 molmol1increase in CO2in this time span is accom-panied by a decrease in O2because combustion and respiration proc
43、esses combine a carbonatom with O2from the atmosphere to produce CO2(Park et al. 2004).The stated argon mole fraction in air has changed from 9340 molmol1at the start of theDonald P. Gatley is a retired consulting engineer, Atlanta, GA. Sebastian Herrmann is a mechanical engineering grad-uate from Z
44、ittau/Goerlitz University of Applied Sciences and is currently a PhD candidate and research assistant at theUniversity of Rostock, Germany. Hans-Joachim Kretzschmar is a professor of technical thermodynamics at Zittau/Go-erlitz University of Applied Sciences, Zittau, Germany.2008, American Society o
45、f Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in HVAC Lemmon and Jacobsen 2004) and the VDI-4670(VDI 2003) models of dry air use similar but not identical atmospheric air models made up of thethree gases: nitrogen, oxygen, and argon. Neither contain CO2. M
46、ost of the remaining modelsinclude 314 molmol1of CO2, which is representative of the middle of the twentieth century. Figure 1. Mass of dry air (19502070).1. Goff and Gratch (1945)2. NOAA/NASA (1976)3. Giacomo (1981)4. Hyland and Wexler (1983)/Hyland et al. (1983)5. Lemmon et al. (2000)6. VDI (2003)7. Lemmon and Jacobsen (2004)8. Park et al. (2004)9. Williams (2007)10. Calm and Hourahan (2007)11. Initial year of 28.966 rounded12. Final year of 28.966 rounded
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